Technical Field
The present invention relates to a self-oscillation circuit for oscillating a vibrator by a positive feedback circuit.
Related Art
A capacitive vibration type pressure/differential pressure sensor or the like includes a self-oscillation circuit for oscillating a vibrator at a resonance frequency. FIG. 8 is a view showing a configuration example of a related-art capacitive vibration type self-oscillation circuit. As shown in FIG. 8, a self-oscillation circuit 500 includes a positive feedback circuit for oscillating a vibrator 511 and a negative feedback circuit for controlling oscillation amplitude of the vibrator 511.
The positive feedback circuit is formed in a loop passing through the vibrator 511, a second fixed electrode 513, an I/V converter 520, an inverting amplifier 530 and a variable-gain amplifier 560 from a first fixed electrode 512. Generally, the vibrator 511 is vacuum-sealed in order to increase a value of Q.
The negative feedback circuit is formed in a circuit passing through an absolute value circuit 540 for detecting an absolute value of a signal outputted from the inverting amplifier 530, an error amplifier 550, and the variable-gain amplifier 560.
In the positive feedback circuit, the vibrator 511 is fixed to a GND potential, and a bias voltage VBIAS is applied to the first fixed electrode 512 and the second fixed electrode 513 via a DC voltage source. At this time, a charge corresponding to capacitance is charged between the vibrator 511 and the first fixed electrode 512, and between the vibrator 511 and the second fixed electrode 513.
In addition to the bias voltage VBIAS, an output voltage VGAO of the variable-gain amplifier 560 is applied to the first fixed electrode 512. The vibrator 511 vibrates in accordance with the potential change of the first fixed electrode 512.
As the vibrator 511 vibrates, the charging and discharging of the charge occurs, and a current output signal from the second fixed electrode 513 is inputted to the I/V converter 520 and is outputted as a voltage signal IVO. The voltage signal IVO is inverted and amplified in the inverting amplifier 530 and is outputted as a voltage signal INVO. The voltage signal INVO is amplified in the variable-gain amplifier 560 and is applied, as the voltage signal VGAO, to the first fixed electrode 512. Such positive feedback circuit allows the vibrator 511 to vibrate at its own resonance frequency.
In the negative feedback circuit, the amplitude of the voltage signal INVO outputted from the inverting amplifier 530 is detected by the absolute value circuit 540. The absolute value circuit 540 can be configured by using a full-wave rectifier circuit or the like. A voltage signal ABSO outputted from the absolute value circuit 540 corresponds to oscillation amplitude of the vibrator 511.
A difference between the voltage signal ABSO and a reference voltage VCONT is detected, as an error signal ERRO, in the error amplifier 550, and the gain of the variable-gain amplifier 560 is changed by the error signal ERRO. In the case of FIG. 8, the gain of the variable-gain amplifier 560 is increased when the amplitude of the vibrator 511 is small and the error signal ERRO is great, and the gain of the variable-gain amplifier 560 is decreased when the amplitude of the vibrator 511 is great and the error signal ERRO is small. As the gain of the variable-gain amplifier 560 is adjusted, the amplitude of the vibrator 511 is normally controlled to be constant.
Patent Document 1: International Publication WO 2011/102062
When such self-oscillation circuit 500 is applied to an apparatus that requires low power consumption, such as, for example, two-wire type instrument, it is necessary to constitute the self-oscillation circuit by an ASIC where it is easy to achieve low power consumption. The reason is that it is difficult to satisfy the low power consumption specification when the self-oscillation circuit is configured by a discrete component.
However, in the related-art self-oscillation circuit 500, a gain of the variable-gain amplifier 560, which is used in the positive feedback, is changed by the output of the negative feedback circuit. Therefore, the positive feedback circuit and the negative feedback circuit have a mutually dependent relationship, and hence, the interface therebetween becomes complicated. Accordingly, a strict adjustment between the positive feedback circuit and the negative feedback circuit is required. For example, when the design changes to the IN converter 520 and the inverting amplifier 530 occur, the design of the variable-gain amplifier 560 should be also changed. This causes an increase in the design man-hours and acts as a barrier to the ASIC.